Homogeneous catalytic O2 reduction to water by a cytochrome c oxidase model with trapping of intermediates and mechanistic insights

An efficient and selective four-electron plus four-proton (4e(-)/4H(+)) reduction of O-2 to water by decamethylferrocene and trifluoroacetic acid can be catalyzed by a synthetic analog of the heme a(3)/Cu-B site in cytochrome c oxidase ((LFeCu)-L-6) or its Cu-free version ((LFe)-L-6) in acetone. A detailed mechanistic-kinetic study on the homogeneous catalytic system reveals spectroscopically detectable intermediates and that the rate-determining step changes from the O-2-binding process at 25 degrees C room temperature (RT) to the O-O bond cleavage of a newly observed FeIII-OOH species at lower temperature (-60 degrees C). At RT, the rate of O-2-binding to (LFeCu)-L-6 is significantly faster than that for (LFe)-L-6, whereas the rates of the O-O bond cleavage of the FeIII-OOH species observed (-60 degrees C) with either the (LFeCu)-L-6 or (LFe)-L-6 catalyst are nearly the same. Thus, the role of the Cu ion is to assist the heme and lead to faster O-2-binding at RT. However, the proximate Cu ion has no effect on the O-O bond cleavage of the FeIII-OOH species at low temperature.